Conventionally, various techniques have been proposed regarding a mobile phone. For example, a mobile phone having two display parts is known, and display devices are known. With a higher performance and a larger-sized screen, a mobile phone has a tendency of increasing its power consumption. In addition, in a dark place such as a movie theater, light emitted from a display surface of a mobile phone is sometimes a hindrance to surroundings. Furthermore, when an incoming call or an electronic mail is received when a user is sleeping and a display of a mobile phone is confirmed, light emitted from a display surface of the mobile phone can be glaring in some cases.